This invention relates to methods and apparatus for measuring the magnitude of spherical indentations made for testing the hardness of the surfaces of solid material, and more particularly to such methods and apparatus which scan and measure spherical indentations with a light beam, the light beam being tilted in one or more planes through the center of the curvature of the indentation as well as perpendicular to the axis of tilting as it is scanned across the indentation being measured.
In my co-pending application Ser. No. 004,325, a method for measuring hardness indentations made in solid surfaces is disclosed in which a concentrated light beam is directed against the testpiece surface in a vertical direction. The light beam is translated parallel to the testpiece surface in at least one direction such that the light spot completely transverses the indentation at least once. During the crossing of the indent, the intensity of the light reflected from the indentation is measured within a predetermined angular range. The light intensity variation occurring when an edge of the indent is crossed, serves as an indicator for marking the initial and the final edge points of the scanned indentation which correspond to the distance between the two edge points of the indentation.
In an especially simple embodiment of the aforesaid application, a light spot is provided which moves along a straight line running through the center of the indent. Then the length of the distance travelled between two intensity variations occurring on the edges of an indent can be measured directly by means of a mechanical or electronic micrometer.
The aforesaid method has certain drawbacks when it is applied to measuring spherical indentations.
An especially designed generally ring-shaped photodetector is required in the aforesaid method. The light-sensitive area of a photodetector which is suitable for the Brinell test becomes relatively large owing to the big angular range of the reflected light, extending from approximately 30.degree. to 80.degree. measured against the vertical. If the measurement is attempted through a spherical indenter, the optical design for the above-mentioned angular range becomes complex. Also, the light that is reflected from the center region of the spherical indentation does not reach the photodetector. In a video display of an X-Y scan of a spherical indentation, the center region would appear as a dark spot of circular shape. Special electronic logic is required to account for the lack of signal when the scanning beam passes through the center region of the indent. Also, the scanning displacements .DELTA.x corresponding to the distances between two edge points are rather small, and so are the corresponding rotation angles .DELTA..phi. of the beam deflecting glass plate or prism. Then too, in most applications where a laser beam is used, a beam expander has to be incorporated in the system in order to get the necessary small spot diameter.